(1) Field of the Invention
The invention relates to a mobile telephone circuit, and more particularly, to a combination of audio and ringing mode wherein both said modes are controlled by the same amplifiers and the same amplitude setting circuits to control the volume of the loudspeaker for both modes.
(2) Description of the Prior Art
The volume adjustment for ringing via a pulse width modulation (PWM) is not very precise. There is a risk of damaging the loudspeaker or the full dynamic range can not be exploited. This results in a high power dissipation which is independent of the volume. For small sound intensity this results in a poor efficiency. There is a risk of over-and undershooting of the amplifier's output signal due to the high slew rate in combination with the inductance of the loudspeaker.
In the current art the volume adjustment is done via a PWM control accepting the imprecise adjustment. There is no solution of the high power dissipation. The risk of over- and undershooting of the amplifier's output signal is solved with external protection diodes.
Several prior art inventions describe the ringer mode of circuits in telephone systems. U.S. Pat. No. 6,252,784 to Dobrenko describes a device for generating an electrical signal (for example a sine wave suitable for a ringer) whose output signal and power supply voltages are correctable through two feedback loops. U.S. Pat. No. 5,541,543 to Arnaud discloses a telephone set fitted with a speech signal loudspeaker also serving as a ringer and more particularly to a device for regulating the power provided to the loudspeaker during a ring signal. U.S. Pat. No. 5,894,212 to Wendt et al. shows a ring signal from a DC power supply using a pair of switches controlled by a pulse width modulation (PWM) circuit.